The present invention relates to the field of methods and devices for the embolization of vascular aneurysms and similar vascular abnormalities. More specifically, the present invention relates to (a) an expansible vascular implant that is inserted into a vascular site such as an aneurysm to create an embolism therein; (b) a method of making the expansible implant; and (c) a method and an apparatus for embolizing a vascular site using the implant.
The embolization of blood vessels is desired in a number of clinical situations. For example, vascular embolization has been used to control vascular bleeding, to occlude the blood supply to tumors, and to occlude vascular aneurysms, particularly intracranial aneurysms. In recent years, vascular embolization for the treatment of aneurysms has received much attention. Several different treatment modalities have been employed in the prior art. U.S. Pat. No. 4,819,637--Dormandy, Jr. et al., for example, describes a vascular embolization system that employs a detachable balloon delivered to the aneurysm site by an intravascular catheter. The balloon is carried into the aneurysm at the tip of the catheter, and it is inflated inside the aneurysm with a solidifying fluid (typically a polymerizable resin or gel) to occlude the aneurysm. The balloon is then detached from the catheter by gentle traction on the catheter. While the balloon-type embolization device can provide an effective occlusion of many types of aneurysms, it is difficult to retrieve or move after the solidifying fluid sets, and it is difficult to visualize unless it is filled with a contrast material. Furthermore, there are risks of balloon rupture during inflation and of premature detachment of the balloon from the catheter.
Another approach is the direct injection of a liquid polymer embolic agent into the vascular site to be occluded. One type of liquid polymer used in the direct injection technique is a rapidly polymerizing liquid, such as a cyanoacrylate resin, particularly isobutyl cyanoacrylate, that is delivered to the target site as a liquid, and then is polymerized in situ. Alternatively, a liquid polymer that is precipitated at the target site from a carrier solution has been used. An example of this type of embolic agent is a cellulose acetate polymer mixed with bismuth trioxide and dissolved in dimethyl sulfoxide (DMSO). Another type is ethylene glycol copolymer dissolved in DMSO. On contact with blood, the DMSO diffuses out, and the polymer precipitates out and rapidly hardens into an embolic mass that conforms to the shape of the aneurysm. Other examples of materials used in this "direct injection" method are disclosed in the following U.S. patents: U.S. Pat. No. 4,551,132--Pasztor et al.; U.S. Pat. No. 4,795,741--Leshchiner et al.; U.S. Pat. No. 5,525,334--Ito et al.; and U.S. Pat. No. 5,580,568--Greff et al.
The direct injection of liquid polymer embolic agents has proven difficult in practice. For example, migration of the polymeric material from the aneurysm and into the adjacent blood vessel has presented a problem. In addition, visualization of the embolization material requires that a contrasting agent be mixed with it, and selecting embolization materials and contrasting agents that are mutually compatible may result in performance compromises that are less than optimal. Furthermore, precise control of the deployment of the polymeric embolization material is difficult, leading to the risk of improper placement and/or premature solidification of the material. Moreover, once the embolization material is deployed and solidified, it is difficult to move or retrieve.
Another approach that has shown promise is the use of thrombogenic microcoils. These microcoils may be made of a biocompatible metal alloy (typically platinum and tungsten) or a suitable polymer. If made of metal, the coil may be provided with Dacron fibers to increase thrombogenicity. The coil is deployed through a microcatheter to the vascular site. Examples of microcoils are disclosed in the following U.S. patents: U.S. Pat. No. 4,994,069--Ritchart et al.; U.S. Pat. No. 5,133,731--Butler et al.; U.S. Pat. No. 5,226,911--Chee et al.; U.S. Pat. No. 5,312,415--Palermo; U.S. Pat. No. 5,382,259--Phelps et al.; U.S. Pat. No. 5,382,260--Dormandy, Jr. et al.; U.S. Pat. No. 5,476,472--Dormandy, Jr. et al.; U.S. Pat. No. 5,578,074--Mirigian; U.S. Pat. No. 5,582,619--Ken; U.S. Pat. No. 5,624,461--Mariant; U.S. Pat. No. 5,645,558--Horton; U.S. Pat. No. 5,658,308--Snyder; and U.S. Pat. No. 5,718,711--Berenstein et al.
The microcoil approach has met with some success in treating small aneurysms with narrow necks, but the coil must be tightly packed into the aneurysm to avoid shifting that can lead to recanalization. Microcoils have been less successful in the treatment of larger aneurysms, especially those with relatively wide necks. A disadvantage of microcoils is that they are not easily retrievable; if a coil migrates out of the aneurysm, a second procedure to retrieve it and move it back into place is necessary. Furthermore, complete packing of an aneurysm using microcoils can be difficult to achieve in practice.
A specific type of microcoil that has achieved a measure of success is the Guglielmi Detachable Coil ("GDC"). The GDC employs a platinum wire coil fixed to a stainless steel guidewire by a solder connection. After the coil is placed inside an aneurysm, an electrical current is applied to the guidewire, which heats sufficiently to melt the solder junction, thereby detaching the coil from the guidewire. The application of the current also creates a positive electrical charge on the coil, which attracts negatively-charged blood cells, platelets, and fibrinogen, thereby increasing the thrombogenicity of the coil. Several coils of different diameters and lengths can be packed into an aneurysm until the aneurysm is completely filled. The coils thus create and hold a thrombus within the aneurysm, inhibiting its displacement and its fragmentation.
The advantages of the GDC procedure are the ability to withdraw and relocate the coil if it migrates from its desired location, and the enhanced ability to promote the formation of a stable thrombus within the aneurysm. Nevertheless, as in conventional microcoil techniques, the successful use of the GDC procedure has been substantially limited to small aneurysms with narrow necks.
Still another approach to the embolization of an abnormal vascular site is the injection into the site of a biocompatible hydrogel, such as poly (2-hydroxyethyl methacrylate) ("pHEMA" or "PHEMA"); or a polyvinyl alcohol foam ("PAF"). See, e.g., Horak et al., "Hydrogels in Endovascular Embolization. II. Clinical Use of Spherical Particles", Biomaterials, Vol. 7, pp. 467-470 (November, 1986); Rao et al., "Hydrolysed Microspheres from Cross-Linked Polymethyl Methacrylate", J. Neuroradiol., Vol. 18, pp. 61-69 (1991); Latchaw et al., "Polyvinyl Foam Embolization of Vascular and Neoplastic Lesions of the Head, Neck, and Spine", Radiology, Vol. 131, pp. 669-679 (June, 1979). These materials are delivered as microparticles in a carrier fluid that is injected into the vascular site, a process that has proven difficult to control.
A further development has been the formulation of the hydrogel materials into a preformed implant or plug that is installed in the vascular site by means such as a microcatheter. See, e.g., U.S. Pat. Nos. 5,258,042--Mehta and 5,456,693--Conston et al. These types of plugs or implants are primarily designed for obstructing blood flow through a tubular vessel or the neck of an aneurysm, and they are not easily adapted for precise implantation within a sack-shaped vascular structure, such as an aneurysm, so as to fill substantially the entire volume of the structure.
There has thus been a long-felt, but as yet unsatisfied need for an aneurysm treatment device and method that can substantially fill aneurysms of a large range of sizes, configurations, and neck widths with a thrombogenic medium with a minimal risk of inadvertent aneurysm rupture or blood vessel wall damage. There has been a further need for such a method and device that also allow for the precise locational deployment of the medium, while also minimizing the potential for migration away from the target location. In addition, a method and device meeting these criteria should also be relatively easy to use in a clinical setting. Such ease of use, for example, should preferably include a provision for good visualization of the device during and after deployment in an aneurysm.